The present specification relates to an electrostatic discharge (ESD) protection device comprising and to a method of making an electrostatic discharge protection device.
The Grounded Gate n-type Metal Oxide Semiconductor Transistor (hereinafter “GGNmost”) is often used as a primary protection element in pad-based ESD-schemes. In some designs the GGNmost is divided into several smaller GGNmosts, which are often referred to as fingers. The individual GGNmosts are connected in parallel.
The fingers in such an ESD protection device can sometimes share a single ring-shaped body contact. Alternatively, each finger may have its own body contact.
A design challenge in such devices is to ensure that all fingers in the GGNmost trigger during an ESD event, so as to ensure that the GGNmost can effectively sink the ESD current without sustaining internal damage. In devices where all fingers share one ring-shaped body contact, one finger may conduct all of the ESD current until a critical current density is reached, at which point a next finger of the device triggers, and so on. In a well-designed GGNmost, this process can take place at a current density that is low compared to the current density at which the GGNmost would fail, so that effective triggering of all fingers may be ensured.
In devices where each finger has its own body contact, triggering of all the fingers of the device may be ensured by increasing the drain ballast resistance to the point that before each finger reaches its fail current density, its voltage drop has already increased beyond the trigger voltage of the other fingers in the device. Accordingly, before each finger fails, another finger of the device will trigger to accommodate the ESD current. A drawback of this particular design is that the device may have a comparatively high voltage drop during an ESD event, which may present a more severe ESD stress to the circuit that the GGNmost is intended to protect.